The boy's innate goodness will withstand the challenges because unless he himself wants to turn evil, [...].
My teacher thinks that commas should set off the word "himself", but I disagree. Who is right, and why?
I think a comma would be wrong there.
(Deletes justification on the basis of it being a rearrangement of "he wants himself to turn evil", which is wrong because "he himself wants Fred to turn evil" is just as valid a sentence. At least I realised that before I posted.)
"Himself" here is actually an intensifier rather than a reflexive pronoun (the World Atlas of Language Structures notes that they are often identical). As such it very much belongs with the noun phrase it is intensifying ("He"), and splitting them with a comma would weaken that relationship.
I don't think commas are necessary here, but they could serve to emphasize that phrase.